The present invention generally relates to packaging machines, and more particularly to packaging machines having seal bars assemblies using electrical resistance impulse heating to form a bond in polymeric material.
Packaging machines are used for sealing a product in an air-tight package. Packages are often constructed of polymeric material. The product to be packaged is placed, by hand or machine, within a bag, a tube or between several sheets of packaging material, then the packaging material openings are sealed closed. Air may or may not be trapped within the package along with the product. A vacuum packaging machine evacuates substantially all the air between the product and the packaging material before sealing. Vacuum packaging is used to reduce package volume and, in the case of perishable product, preserve freshness.
Various techniques are utilized for bonding together layers of polymeric packaging material. One device to effect polymeric bonding is an impulse heating apparatus, or seal bar assembly. The surfaces of polymeric material to be sealed are placed adjacent to each other and in contact with the seal bar assembly. Application of pressure and sufficient heat partially melts and bonds the surfaces of the polymeric material together to form a seal. Cooling stabilizes the bond.
Conventional seal bars are generally constructed of a metallic block having smooth face mating surface. A flat, electrically resistive wire is disposed on a seal bar assembly smooth face surface. Another bar, opposing the seal bar, applies pressure to hold the packaging material in contact with the seal bar. Heat is generated by passing sufficient electricity through the sealing wire for a period of time (dwell time) until the wire reaches a desired temperature. Heat is transmitted from the sealing wire to the packaging material to thermally bond the surfaces of the polymeric material to each other. Electricity is removed from the seal wire and the package is allowed to cool, stabilizing the bond. Contact pressure is relieved and the sealed package is removed from the seal bar to complete a package sealing cycle. The conventional sealing wire is a discrete electrically resistive heating element, but may also be integrally formed onto the surface of the seal bar.
Reducing package sealing cycle time increases efficiency and reduces the cost of packaging. One method to reduce package sealing cycle time is to cool, and thus stabilize, the bond more quickly. Forming the seal using only the minimum heat necessary is desirable, so that time to dissipate heat in excess of bond-forming heat is avoided. However, if too little heat is input, bond quality will decline. Quickly conducting heat away from a completed bond also decreases cool-down time. The seal bar preferably acts as a heat sink to remove heat generated by the sealing wire from the area of the bond after the seal is formed. Therefore, the seal bars are constructed of material possessing efficient thermal conductivity properties.
The conventional seal bar is constructed of aluminum. Aluminum is relatively easy and inexpensive to manufacture. Aluminum possesses desirable heat conduction properties and can be machined to a smooth surface, impervious to moisture and bacteria. Heat is conducted via the seal bar to a cooler ambient environment or to a heat removal fluid (liquid or gas) passed through the seal bar itself.
Conventionally, a layer of insulating material is disposed between the sealing wire and the electrically conductive metallic (i.e., aluminum) seal bar to prevent shorting of the electrical heating circuit. One common method to provide such electrical insulation is to dispose a thin, adhesive-backed TEFLON tape adhered to the metallic seal bar smooth face surface. The sealing wire is subsequently disposed onto the non-adhesive side of the TEFLON tape.
A thin layer of release coating material is conventionally disposed over the outer surface of the sealing wire to prevent the polymers of the polymeric material from sticking to the hot sealing wire, commonly known as xe2x80x9cpoly prick.xe2x80x9d Poly prick decreases the integrity of the polymeric packaging material bond due to the loss of polymeric material extracted by the sealing wire from the joint area.
Additionally, over time, the sealing wire heat transfer capability is reduced in isolated regions along the sealing wire due to the insulating presence of melted polymeric materials adhering to the wire. Bond integrity is subsequently reduced in these regions if insufficient heat is transferred to the packaging material in these areas. Alternatively, if sealing wire temperatures are increased to compensate for the isolated areas of seal wire insulation due to poly prick, un-insulated areas of the seal wire can overheat, burning or otherwise damaging the polymeric material. One conventional method of preventing poly prick is to place another layer of TEFLON tape over the hot sealing wire to form another smooth face surface on the seal bar. The TEFLON is disposed, adhesive side towards the sealing wire, between the sealing wire and the polymeric packaging material to be bonded.
The sealing wire is conventionally constructed from a metal or metallic alloy such as silver or nickel alloy. The long, flat sealing wire thermally expands and contracts as it heats and cools during each impulse heat sealing cycle. The cyclic expansion/contraction stress the sealing wire.
Areas along the sealing wire experiencing differential heat transfer rates due to the presence of polymeric material on the sealing wire from poly prick. Polymeric material on the sealing wire insulates the sealing wire reducing heat transfer from the sealing wire to the packaging material, creating sealing wire hot spots. xe2x80x9cHotxe2x80x9d and xe2x80x9ccoldxe2x80x9d regions exist along the sealing wire due to difference in thermal transfer characteristics. Additional thermal stress exists between the xe2x80x9chotxe2x80x9d and xe2x80x9ccoldxe2x80x9d regions along the sealing wire.
The thermal expansion/contraction cycles also cause the sealing wire to mechanically creep during each cycle. This sealing wire movement creates wear of the insulating TEFLON tape as the sealing wire moves relative to the TEFLON tape. If the TEFLON tapes wear through, electrical insulation is lost, the sealing wire shorts out and losses ability to generate heat, and thus losses the ability to form a bond in the polymeric material along the entire length of the desired seal.
Along with the mechanical movement of the sealing wire imparted to the TEFLON tapes, the TEFLON tapes also undergo thermal expansion/contraction cycles as heat from the sealing wire is imparted to the layers of TEFLON tape by conduction. The TEFLON tapes frequently bunch during sealing operations using conventional seal bar and seal bar mating block assemblies.
TEFLON tape bunching is deleterious to seal quality. TEFLON tape bunching creates peaks and valleys in the surface of the seal and mating bars, resulting in uneven application of pressure. Once the TEFLON tape bunches, the adhesive-backing releases and the TEFLON tape can overlap itself. Furthermore, conduction of heat from the sealing wire through the TEFLON tape disposed between the sealing wire and the polymeric material varies between the peaks, valleys and increased thickness of overlapped areas of the bunched TEFLON tape. Due to the varying degrees of contact between the polymeric materials and the seal bar, and the varying heat transfer characteristics of the TEFLON tape, bond strength varies along the joint, and quality is thus reduced (seal integrity is only as good as the weakest point along the seal).
Bunching of one TEFLON tape tends to mechanically cause the other TEFLON tapes in close contact to also bunch up. Therefore, bunched TEFLON tapes must be removed and replaced along with the sealing wire disposed therebetween. TEFLON tape replacement requires removing the offending seal bar assembly from service, reducing efficiency and increasing costs. Package machine down time, maintenance labor costs and replacement part costs (TEFLON tapes and sealing wires) further increase costs associated with packaging machines employing conventional seal bar assembly and seal bar mating assembly. A seal bar assembly cycling 30 to 40 times per minute, 16 hours per day typically requires TEFLON tape and sealing wire replacement 2 or 3 times per day.
However, due to the need to heat the polymeric material through a layer of TEFLON tape, and the aforementioned additional heat transfer problems of bunched TEFLON tape, often conventional seal bar assemblies are upgraded at significant cost to use dual sealing wires. Dual sealing wires are capable of generating increased heat levels, i.e., the sealing wire can experience a broader range of operating temperature. The severity of thermal expansion/contraction cycles is likewise increased across a broader temperature range, leading to more rapid TEFLON tape failures.
Dual sealing wire installation require approximately double the energy input as the single sealing wire application since the resistance of the heating element is effectively doubled. Upgrading from a single to a dual sealing wire seal bar assembly typically includes the expense of supplementing the power supply capability of the packaging machine to accommodate the increased seal bar assembly power usage. Operating costs are proportionately higher with higher energy usage. Maintenance costs, replacement part costs and labor costs are all increased as well due to the effects of a higher magnitude thermal expansion/contraction cycling.
While capital, direct operating and maintenance costs are typically less for a single sealing wire versus a dual sealing wire configuration, seal quality is lower for a package sealed with a single sealing wire. Increased costs can result due to increased repackaging or spoilage costs from faulty seals. Labor to repackage, package material costs and lost capacity to package xe2x80x9cnewxe2x80x9d product all contribute to the costs associated with faulty sealing operations.
Dual sealing wire configurations essentially form two adjacent bonds. In one typical conventional embodiment, the dual electrical resistive heating elements (sealing wires) are energized in series at a higher voltage, rather than in parallel at a higher current. This requires separation between the adjacent heating wires/ribbons since some voltage difference between the individual dual heating wires/ribbons exists everywhere along the seal bar. An electrical short circuit and loss of some heating area along the seal bar assembly results if the dual heating elements are located too close together.
When liquid is present in the bond area between the dual sealing wires, as is the case for moist products such as meat, the sealing wires must supply the additional energy necessary to heat the liquid, along with the polymeric material, to a temperature sufficient to form a bond. Significant energy is lost to vaporize the liquid since the requisite bond-forming temperature typically exceeds the boiling point of the trapped liquid. As the liquid vaporizes, the gas under pressure becomes trapped between the dual bonds, tending to force the layers of polymeric material apart.
A low resistance electrical lead is coupled to each end of the sealing wire providing a path to supply electrical power to the seal bar assembly. The electrical leads are provided with a spring loaded clip at one end for quick coupling to a power source terminals located on the packaging machine. In the conventional single sealing wire seal bar assembly, the seal wire spans the length of the seal bar. One short electrical lead is coupled to the sealing wire end located at each end of the seal bar. A conventional packaging machine provides a power source terminal within a short distance from each end of the seal bar.
When a seal bar assembly is upgraded to provide a dual sealing wire configuration however, the two sealing wires span the length of the seal bar and are electrically coupled together on one end of the seal bar. The dual sealing wires are effectively connected in series with each other. At end of the seal bar opposite the junction between the dual sealing wire elements, an electrical lead is coupled to each of the remaining ends of the dual seal wires. Therefore, one short and one longer electrical lead are necessary to reach the conventional packaging machine power source terminals. The longer electrical lead is approximately 16 inches long and more susceptible to damage such as nicks, cuts, and pinching under the seal bar.
FIG. 4 illustrates a conventional seal bar mating assembly having a rigid base and preferably, a substantially planar contour is used to apply pressure to the packaging materials in order to keep them in contact with the seal bar assembly during sealing operations. A rubber layer is disposed onto a rigid base to form the mating block. A thin, adhesive-backed TEFLON tape layer is adhered onto the rubber layer to form a mating surface of the mating block. A reciprocating means is used to bring the smooth face surface of the seal bar assembly towards the mating surface of the seal bar mating assembly during sealing operations with the polymeric material to be sealed disposed therebetween. The seal bar mating assembly TEFLON tape can also bunch preventing the application of uniform pressure to the polymeric material and accompanying decrease in bond quality.
In view of the above, there is a need for a seal bar assembly and seal bar mating assembly which do not require the use of TEFLON tapes. Eliminating the use of TEFLON tapes and the maintenance costs associated with bunching thereof and electrical shorting of sealing wire is desirable. An improved seal bar assembly is preferably compatible with existing packaging equipment, utilizes a single sealing wire to eliminate power supply upgrade capital costs, reduces energy costs and yields consistent, high quality bonds in polymeric materials.
The present invention generally relates to packaging machines, and more particularly to packaging machines having seal bars assemblies using electrical resistance impulse heating to form a bond in polymeric material.
A seal bar assembly is disclosed comprising a seal bar having a smooth face surface, a length of electrical resistance heating element, and a layer of electrical insulation disposed around the perimeter of a center portion of the length of the electrical resistance heating element, the length of insulated electrical heating element disposed upon the seal bar smooth face surface.
In one embodiment, the electrical resistance heating element is preferably flat, and more preferably configured as a double seam band. The electrical resistance heating element is preferably coated with TEFLON and/or SILVERSTONE except for its ends which are preferably copper plated. Low resistance electrical leads are coupled to the ends of the electrical resistance heating element.
In one embodiment, the seal bar is preferably formed from virgin TEFLON, or alternately metal coated with TEFLON or SILVERSTONE. In one embodiment, the electrical resistance heating element is mounted onto the seal bar using a constant tension mechanism.
A seal bar mating assembly is disclosed using a layer of silicon rubber to eliminate the need for using TEFLON tape. A improved packaging machine using the seal bar assembly of the present invention is provided.